1. Field of the Invention
The present invention relates generally to a shorter wavelength light generating apparatus for stably generating shorter wavelength light utilized for high-density optical disk system, and more particularly to a shorter wavelength light generating apparatus in which a fundamental wave radiated from a semiconductor laser is converted into a second harmonic wave.
2. Description of the Related Art
Coherent green light and coherent blue light are required to read pieces of information stored in an optical disk in a high density or to process pictures. The coherent green light and the coherent blue light are produced by efficiently converting a fundamental wave radiated from a semiconductor laser into shorter wavelength light such as a second harmonic wave with a wavelength converting device. Coherent light radiated from a semiconductor laser consists of a fundamental wave. Also, harmonic waves such as a second harmonic wave have been known. A wavelength .lambda..sub.f of the fundamental wave is longer than those of the harmonic waves, and each of wavelengths of the harmonic waves is defined as a multiple of the wavelength .lambda..sub.f of the fundamental wave. For example, the wavelength .lambda..sub.h of the second harmonic wave is half of the wavelength .lambda..sub.f of the fundamental wave. Therefore, the coherent green light or the coherent blue light is generated from infrared radiation by converting the fundamental wave having a wavelength of 860 nm into the second harmonic wave having a wavelength of 430 nm in the wavelength converting device.
In this case, to efficiently output shorter wavelength light from coherent light generated in the semiconductor laser, a transverse mode of the shorter wavelength light is required to be distributed in a Gaussian distribution, the shorter wavelength light diffracted in a diffraction-limited range is required to be focused, and the shorter wavelength light having an output power of over 1 mW and a stabilized wavelength is required to be stably output.
To obtain the shorter wavelength light having the output power of over 1 mW, an optical waveguide type of wavelength converting device, a bulk element type of wavelength converting device, and KNbO.sub.3 have been conventionally utilized. Those devices and KNbO.sub.3 are respectively formed of a large bulk of crystal having a non-linear polarization constant. The optical waveguide type of wavelength converting device has alternate rows of reverse and non-reverse polarization layers arranged at regular intervals in an optical waveguide, and a fundamental wave of semiconductor laser is converted into a second harmonic wave in the optical waveguide according to a quasi-phase matching (Yamamoto et. al. Optics Letters, Vol. 16, No. 15, 1156 (1991)). Also, the bulk element type of wavelength converting device has alternate rows of reverse and non-reverse polarization layers arranged at regular intervals, and a fundamental wave of semiconductor laser is also converted into a second harmonic wave according to the quasi-phase matching.
Also, a shorter wavelength light generating apparatus in which the wavelength of the shorter wavelength light generated is changeable is required for a measuring field such as high-resolution gas spectroscopy and atmospheric pollution monitoring.